Despite the diversity of the nature, morphology, characteristics and function of isolated animal cells, it is presently generally admitted that these cells are derived from a single cell population, termed “stem cells”. Unlike more mature or differentiated cells, stem cells are capable of self-regeneration but may also divide into progenitor cells that are no longer pluripotent nor capable of self-regeneration. These progenitor cells divide repeatedly to form more mature cells, which eventually become terminally differentiated to form various mature cells. Thus the large number of mature cells is derived from a small reservoir of stem cells by a process of proliferation and differentiation.
In the particular domain of blood vessels, one promising solution to the problems associated with synthetic-based replacement tissues, such as vascular grafts, is to assemble blood vessels in vitro using only the patient's own cells and then re-implant them into the patient. In theory, tissue-engineered blood vessels should provide mechanically stable vessels built only from autologous tissue, therefore generating no immune responses. Tissue engineering has been used successfully in the past to build less complex structures such as skin, but has had only relative success with other three-dimensional tissues and organs such as tissue-engineered blood vessels. Common problems associated with three-dimensional engineered tissues include the complexity of reconstruction, the lack of structural integrity and mechanical strength, and the need for biologically active tissues. This is a particular problem for tissue-engineered blood vessels, since these vessels will be subjected to significant mechanical loads both from blood pressure (which may be abnormally high in patients with heart disease), as well as from the relative motion between the anchoring sites of the vessel. Moreover, the tissue-engineered blood vessels must demonstrate sufficient stability and tear resistance to allow surgical handling and implantation, and require a biologically active endothelial layer.
An important cause of these problems seems to be associated with the sources of the cells used to prepare engineered blood vessels or other replacement tissues. In fact, most methods available at the moment allow the isolation of a maximum of two cell types from a single tissue, which should be preferentially autologous. Also, these cells, alone or mixed with other cell types, are generally mature and well differentiated, which constitutes a limiting factor when a certain level of flexibility and adaptability is necessary in constructing an engineered replacement tissue.
Stem cells mature into progenitor cells and then become lineage committed, that is, incapable of maturing into all of the different lineages which the stem cell is capable of producing. Highly purified populations of stem cells currently find use in the long-term repopulation of particular body systems. Purified progenitor cells of individual lineages would find use only in transiently repopulating or augmenting the various lineages. As progenitors are not believed to be self-regenerating, the repopulation or augmentation would be limited, for example, to short-term tissue-specific reconstitution.
The field of tissue engineering uses living cultured human or animal cells from various sources to reconstruct functional tissues and organs for experimental and therapeutic purposes.
Medical applications of umbilical haematopoietic cord stem cells are becoming well known both in and out of the medical community.
As a result of the successful clinical experiences and the large number of people who have benefited from the medical uses of cord haematopoietic stem cells to date (or who will benefit from cord stem cell uses in the future), different cord stem cell preparations are presently under investigation.
The use of the pluripotentiality of lineage-committed progenitor cells circumvents many of the problems that would arise from the transfer of mature cells. However, such progenitor cells may have to be separated through carrying out parallel methods upon different sources of living tissues. Separation requires identification of the cells and characterization of phenotypic differences that can be utilized in a separation procedure. In addition, the methods known at this time for preparing replacement tissues still have to be carried out with cells originating from heterologous sources, therefore creating problems of compatibility in using such replacement tissues.
However, the cells originating from the umbilical cord tissue remain underused both as progenitor cells, and in complex cell compositions for the preparation of replacement and engineered tissues.
Considering the state of the art described herein, there is still a need for single sources of multiple cell types for the preparation of progenitor cells or replacement cells or tissues. Moreover, there still exist no complex cell compositions capable of being completely compatible when reintroduced into a human or animal body.